Work (harmless) pranks thread

What has everyone else done while at work that's funny? Here's one I plan on doing tomorrow just to see what happens:

I'm going to unplug my victim's mouse and see just how it takes to get one of the IT guys out there to correct the problem.
I did something similar yesterday by simply unplugging the electric pencil sharpener. I'm not sure how long the maintenance request took, but I'm sure it wasn't very late in the morning !